1. Field of the Invention
The present invention relates to a photodetector and, more particularly, to an avalanche photodetector (APD) adapted for high-speed long-distance optical fiber communication.
2. Description of Related Art
Recently, APDs have become the leading device in photo-receiver market in 2.5 GHz or 10 GHz bandwidth for optical communication. Compared with fast p-i-n photodetectors, APD can provide higher sensitivity and gain in the desired bandwidth. As compared to photo-transistors (HPTs) with the same gain, APDs are preferred in its speed potential and short falling time of impulse response, such that they are more practical in eye-diagram test and commercial optical communication than HPTs do. When a high-speed photodetector is developed in a market of high-speed long-distance optical fiber communication, two factors of high output power-bandwidth product and high efficiency-bandwidth product need to be met. However, the traditional APDs cannot meet with the two factors at the same time.
The electrons and holes in III-V material have almost the same ionization coefficients when multiplication avalanche occurs. By contrast, in a silicon material, electrons are the dominated carriers for multiplication process (i.e. over 99% of carriers), such that an APD with Si-based substrate has better performance in bandwidth, noise-elimination, and gain performance than an APD with III-V based substrate does. However, the APD with Si-based substrate does not response to any radiation in any wavelength of optical fiber communication due to the relative large bandgap energy of silicon. To overcome this problem, an InGaAs absorption layer is suggested to attach to a Si multiplication layer by wafer bonding, so as to form a separate absorption multiplication APD (SAM APD) (e.g. InGaAs fused Si APD in U.S. Pat. No. 6,465,803). However, such a wafer-bonding device has several disadvantages like high cost, high dark current, and brittleness caused by different heat expansion coefficients between Si and InGaAs. Therefore, another APD structure, applied in optical communication, having a Si multiplication layer and an absorption layer with SiGe alloy is disclosed in U.S. Pat. No. 6,457,107 which provides a novel long wavelength APD with a SiGeC absorption layer. However, such a structure still has high operating voltage and high device capacitance. Further, SiGeC cannot be grown directly into a thicker absorption layer in practical production and application.
Therefore, it is desirable to provide an improved APD to mitigate and/or obviate the aforementioned problems.